Chromium base alloy



United States Patent 3,208,847 CHROMIUM BASE ALLGY James E. Fox, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 26, 1962, Ser. No. 182,645 3 Claims. (Cl. 75176) This invention relates to chromium base alloys and more particularly, to chromium-yttrium base alloys with additions of aluminum and tungsten or rhenium to impart improved properties.

A refractory metal which has attractive possibilities as the base for high temperatrue alloys is chromium. Applications for a strong alloy which has elevated temperature oxidation resistance includes articles used in power producing or propulsion apparatus operating at elevated temperatures such as 23 0O2500 F.

It has been reported previously that additions of yttrium to chromium improves the air oxidation of chromium base alloys.

It is a principal object of this invention to provide a chromium base alloy of improved oxidation resistance and which has good high temperature strength.

Another object is to provide an improved chromium base alloy including yttrium and aluminum for improved oxidation resistance along with the elements tungsten or rhenium to provide improved strength. I

These and other objects and advantages will be more readily recognized from the following detailed description and examples which are meant to be exemplary of rather than limitations on the scope of the present invention.

Briefly, the improved chromium base alloy of the present invention consists essentially of, by weight, 0.2 0.7% yttrium; 25% aluminum; one of the elements tungsten and rhenium, the tungsten being in the range of 9-12% and the rhenium being in the range of 11-16%; with the balance chromium.

The alloy of the present invention has been found to provide high strength and oxidation resistance for applications up to about 2500 F. By modifying a chromiumyttrium alloy with additions of aluminum, improved oxidation resistance was obtained. By further modifying such alloy with the additions of certain refractory metals, improved strength was obtained.

The following Table I gives the weight percentages of alloys melted and tested in an evaluation of the present invention:

Table I [WEIGHT PERCENT (BALANCE 01*)1 Cast Alloy Y Al W Re H(1rRdn)ess The yttrium content referred to herein in connection with the alloy composition is the yttrium retained in the alloy and not the yttrium added during the melting of the 3,203,847 Patented Sept. 28, 1965 alloy. Frequently there are relatively large variations between the amount of yttrium added and that retained in the alloy because of yttriums gettering effect.

It is to be noted in the table that with the inclusion of as little as about 1% tungsten and a small amount of aluminum, the hardness, which is related to the strength, is significantly lower than the other alloys. Similarly, alloy 56 shows that the inclusion of only a small amount of rhenium results in a softer and weaker alloy. Thus, because alloy 42 shows improved hardness with a tungsten addition of about 2 weight percent and because alloy 54 shows a strength improvement with the addition of about 6 weight percent rhenium, from a strength consideration alone alloys including substantially less than these amounts would be relatively weak at elevated temperatures.

The following Table II represents 2500 F. oxidation data from tests of alloys Within and outside the scope of this invention. As listed in Table II, the weight change (A wt.) has the following meaning: for gross-the change in weight of the specimen before testing and the specimen plus any residue, attached or detached, after testing; for net"the change in weight of the specimen before testing and the specimen including any oxidation film tightly adherent to the specimen surface, after testing; and for diif.--the total weight difference between the gross and net weights.

A large gross change indicates a substantial accumulation of oxidation product, and a large net change indicates that a relatively large amount of the oxidation product was detached, such as through flaking, during oxidation testing. However, even if the gross change is high, a relatively small difference between gross and net means that the oxidation product is adherent and therefore the alloy has good oxidation resistance. Such is the case with alloy 43 which has a tightly adherent surface portion formed during exposure to air at elevated temperatures.

It is to be noted that alloys 41, 68, 69, 72 and 73 exhibit a relatively large difference between gross and net weight change with alloys 68, 69, 72 and 73 having unusually large net losses. Thus when aluminum is not included, 2500 F. oxidation resistance is seriously affected.

Table II 2,500 F. Oxidation Alloy Time A wt. (mg/cm!) DitI.

(hr.) Gross Net 11.6 l.0 12.6 25 29. 5 38. 8 G8. 3 9O 7. 8 14. 7 22. 5

Specimens used in the oxidation tests were 0.25 inch thick disks cut from one inch diameter ingots. Specimens were held in A1 0 boats and placed in static air furnaces. The gross weight changes were measured at 25 hour intervals and the net weight changes were determined after completion of the tests.

The alloy of the present invention combines strength, as indicated by the cast hardness in Table I and the tensile 

1. AN IMPROVED CHROMIUM BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT: 0.2-0.7% Y; 2-5% AL; ONE OF THE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF W AND RE, THE W BEING IN THE RANGE OF 9-12% AND THE RE BEING IN THE RANGE OF 11-16% WITH THE BALANCE CHROMIUM, THE ALUMINUM BEING IN THE RANGE OF 4-5% WHEN W IS SELECTED AND 2-3% WHEN RE IS SELECTED. 